The story of two Korean orphans adopted by very different American families, Anne Tyler's 17th novel at first seems to be quintessential Tyler fare, embodying as it does her favourite themes of home and homesickness, belonging and individual freedom. All the ingredients are here: the Baltimore setting, the overbearing wives and spineless spouses, and the woman who suddenly finds herself a stranger in her own life.

But the arrival of two foreign babies brings new, perhaps more timely, considerations: cultural differences, tolerance and assimilation, and, above all, the idea of what it means to be an American. To call the novel political would be overstating the case: Tyler is, after all, the celebrated chronicler of everyday America, often compared to Jane Austen not only for her wit and lightness of touch, but for the myopic absence of any world view. However, though 9/11 is alluded to only glancingly, it seems not even she has been completely immune to the events of the last few years.

Recalling her earlier novel, Back When We Were Grown Up, about a family whose lives are overrun by entertaining, Digging to America is constructed around a succession of superbly comic family gatherings, the first at Baltimore airport. Bitsy and Brad Donaldson (sporting MOM and DAD badges) have brought their entire clan, two sets of GRANDMAs and GRANDPAs, UNCLEs and COUSINs all attached to video cameras to record the arrival of the wholesome Jin Ho. They make such a performance that we hardly notice a younger couple, accompanied only by one grandmother-to-be, completely absorbed in the arrival of their own baby - the more fragile Sooki.

It seems the stage is set for a comedy of misplaced babies and, predictably, the girls grow up to be more suited to the other's parents, but Tyler is far too sophisticated a writer for such a neat storyline. Instead, the novel is more concerned with the awkward friendship that develops between the two families: the Donaldsons, despite their ethnic affectations, as American as a couple of Big Macs; and the Iranian-American Yazduns who, beneath their Baltimore accents and fashionable outfits, still consider themselves foreigners. Sooki's grandmother Maryam came to America as a young bride, but feels "still and forever a guest, on her very best behaviour". The Yazduns change Sooki's name to Susan, dress her in jeans and wean her on to Cheerios; while Jin Ho (who later insists on being called Jo) is forced to wear Korean costumes, listen to Korean folk music and drink soy milk.

With her lectures on childrearing, belief in "positive enforcement", and horror of orange squash, Bitsy is as clumsy and squirmingly awful as one of her home-made rug-like dresses. She has spent her life doing bitsy things - a bit of weaving, teaching yoga, poetry - but now she has found her vocation as Earth Mother. But it is not just her new age whimsies that make her the target of Tyler's uncharacteristically unforgiving satire. Her bossy sanctimoniousness and disregard for personal boundaries make her a caricature of the prevailing American stereotype. It is Bitsy, of course, who comes up with the idea of the "arrival party" to mark the anniversary of the girls arriving in the Land of the Free - cheerfully unaware of the irony of kitting the girls out like a pair of Korean dolls, only to present them with a stars-and-stripes frosted cake.

Despite the endless celebrations, tragedy is always tiptoeing at the edges, like an uninvited guest. And as in Tyler's previous novel, The Amateur Marriage, the most dramatic events occur offstage - with the exception of one excruciatingly public scene, which takes place on the Donaldsons' front lawn. All these parties provide Tyler with the set pieces at which she so excels - although after the third or fourth farcical arrival ceremonies, the reader begins to tire of them as much as some of the family members. This also contributes to the sense in some of Tyler's more recent fiction that the parts, deliciously funny and sharply observed, are more satisfying than the whole.

Digging to America will do little to dissuade Tyler's detractors, for whom, as one critic acidly put it, she has become "America's foremost Nutrasweet novelist". But there's more spice and bitterness here. Like Maryam, Tyler seems to be angry, ashamed of the blundering superiority of her compatriots - at least as they appear to the rest of the world: "their blithe assumption that their way was the only way!" But like Maryam, she lets them off the hook in the end. Just when the Donaldsons seem to have finally overstepped the mark, their relentless cheeriness triumphs and it's party time once more. Tyler is a sucker for happy endings. Yes, we can be ghastly, she seems to be saying, but we mean well really - and we sure know how to enjoy ourselves.